1. Field of the Invention
The invention is a process for fabricating electronic interconnect circuits, such as flexible circuits having etched or plated features on both sides of a dielectric substrate, especially IC packaging and tape ball grid array type (TBGA) circuits.
2. Description of the Related Art
Conventional imaging and exposure techniques used in producing fine pitch flexible circuits require the use of a discrete photomask which has opaque regions which define the image that will be exposed on the circuit substrate. Registration and exposure using a discrete photomask is a slow process due to the mechanics associated with indexing the web as the mechanism which holds the photomask is opened and closed. Circuit configurations requiring two side imaging require near exact alignment between the features on the opposing sides of the substrate. An imaging process using discrete photomasks requires that the two photomasks be in hinged relation to each other such that they can be moved away from the web to allow for the indexing of the web. This opening and closing operation is an operation that can contributes to misalignment between the images on the two sides of the substrate as there is typically no active alignment system used to verify registration.
U.S. Pat. No. 5,637,426 discloses a method for forming a resist pattern on substrates such as a printed circuit boards, semiconductor substrates, and transparent glass or plastic substrates. The patterned resist layer can be used for defining features such as circuitry layers or solder resist patterns. The method entails forming a resist layer on an object, placing a transparent mask substrate on the resist covered object, establishing a mask pattern directly on the transparent mask substrate with an ink jet printer, exposing the resist to light with intervention of the mask, removing the transparent mask substrate, and then developing the resist. The use of an ink jet printer and the absence of two side processing of circuits in the process of Uchikawa are key departures from the present invention. No means or utility relating to the formation or processing of features residing on opposing faces of an object, is disclosed. In particular, Uchikawa makes no mention of a means for registering mask patterns which are formed on opposing sides of a circuit substrate. Similarly, no method is disclosed for meeting registration requirements between dependent features residing on opposing sides for ensuring that acceptable electrical performance is obtained. A key limitation in the claims of Uchikawa is that the substrate being processed has a plurality of through holes. In the case of a circuit, this would mean that the Uchikawa process is not being used to produce the through holes.
U.S. Pat. No. 4,853,317 discloses a method for serializing printed or flexible circuit boards utilizing an ink jet printer. The method is stated to be useful in that it allows individual boards to be serialized with unique indicia, but without requiring a separate photomask or printing plate for each desired serialization. The method entails providing a flexible or printed circuit board, printing serialization information directly onto the photoresist layer, placing a photomask with a circuit patterning image over the photoresist, irradiating the photoresist and finally developing the photoresist. The use of an ink jet printer and the absence the two side processing of circuits in the process of Hayes are key departures from the present invention. As with Uchikawa, for image resolutions required for practical utility of the present invention, ink jetters are known to be incapable of imaging at rates comparable to the mask pattern producing means of the present invention. Furthermore, no means for two side processing is taught nor suggested nor does the utility (serialization) disclosed inherently require two side processing.
In view of the above, and because the speed for conventional imaging is approximately 0.30 to 1 meter/minute, it would be desirable for a process to allow high speed fabrication and two side processing.
The process of the current invention utilizes image patterning and exposure sequence steps having higher speed than are currently possible with conventional pattern imaging and exposure techniques. This high speed patterning and exposure results in reduced manufacturing cost and the ability to economically manufacture smaller quantity production runs. The process is applicable to both additive and subtractive fabrication processes.
In the present invention, a high speed printing method such as gravure, flexographic, or offset is used for producing a desired image directly on the coversheet of a photoresist clad dielectric substrate, or alternatively, to print directly on the photoresist itself. This type of patterning format is distinctly different from conventional imaging where a discrete photomask is used for generating the pattern. Rather than the pattern being provided on a transparent substrate such as glass, the image is printed with ink on the photoresist or on the polymeric substrate on which the photoresist is supplied.
When image exposure and development are done in continuous process with the printing, the combined speed of printing and imaging for the present invention is from approximately 1.8 to about 3 meters per minute. However, the exposure speed is limited by the sensitivity of the photoresist and the intensity of the light source. Depending on the type of light source and sensitivity of the photoresist, exposure rates are approximately 0.05 meters/second to about 0.16 meters/second. Thus, the imaging is the rate-limiting step, i.e., if the processes are not combined, independent high speed printing rates are much faster. The printing rates for the various high speed printing methods mentioned can be as high as 46 meters/minute.